The present invention relates to overload protectors for preventing excessive radio-frequency energy, applied to an input, from reaching an output.
There are many applications in which a delicate input circuit must be protected from excessive voltages, current or power. For example, certain field effect transistors and other devices cannot tolerate excessive voltage at their inputs. Other examples include the input to certain receivers. For example, in aircraft weather radar systems, the input to a mixer can be damaged by excessive power emanating from nearby radio sources or reflected power originating from the radar itself. Radar systems are especially susceptible to overloading because they incorporate an antenna serving the dual function of transmitting relatively high energy pulses and receiving very faint signals of the same form. In the event such an antenna is damaged or broken off, it is likely that these high energy pulses will not be safely transmitted but will be coupled directly into the input of the radar receiver.
Such radar systems have been very difficult to protect. Known methods of protecting the radar have included rotating the weather radar antenna so it would not receive damaging signals from reflections or from other nearby, operating radar systems until the aircraft has left the heavily trafficked area. One unsuccessful method for protecting the radar system is to turn off its power. However, even when power has been removed, the radar front end and its sensitive components are still exposed to receipt of damaging energy from nearby high frequency sources.
A known technique for protecting a sensitive input is by shunting the input with one or more stages of pin diodes. A relatively large, radio frequency signal placed across the pin diodes will forward bias them. The forward biasing will persist because of the capacitance of the diode. This approach is inherently limited since this power must be absorbed by the pin diode which must therefor have a high power rating. Consequently, the pin diode tends to be rather slow and will allow substantial power to reach the protected circuit before the diode becomes effective. Moreover, pin diodes do not provide a perfect short but will only reduce the dynamic shunting impedance across the input of the protected circuit.
With high frequency circuits it is often desirable and practical to take advantage of the relatively short wavelengths of signals propagating through a circuit. For example, a transmission line may have one end shorted or open but depending upon the effective electrical length of the transmission line, the other end can appear as either an open or short circuit. Similarly, depending upon the spacing of ports on a transmission line, either complete or no coupling will occur between ports. This phenomena is used in circulators, directional couplers and hybrid couplers. These various effects can be produced with waveguides, cables, microstrips, strip lines and through known equivalent circuits that simulate the effect of a transmission line.
Accordingly, there is need for a device for protecting a delicate circuit by interrupting a higher energy power flow more quickly and more completely than has been possible with systems of the prior art.